In the related art, various types of liquid processing apparatus have been known which perform a liquid-processing of a substrate (“wafer”) such as cleaning, etching, plating and developing of the substrate by supplying a processing liquid to a front surface or a back surface of the substrate while holding and rotating the substrate in a horizontal state. See, for example, Japanese Patent Application Laid-Open No. 2009-94525 which discloses a single type liquid processing apparatus that processes substrates one by one. That is, a substrate is rotated while being held in a horizontal state by a spin chuck, and a processing liquid is supplied to the surface of the substrate while being held and rotated by the spin chuck. In the single type liquid processing apparatus, a technology has been known in which a fan filter unit (“FFU”) is installed above a processing chamber and a gas such as nitrogen (N2) gas or clean air is sent from the FFU into the processing chamber in a down-flow mode.